The standard VDE 0126-1-1 requires that an inverter separate itself from the grid within specified switch-off terms on the basis of the effective value of a differential current suddenly occurring over its grid connections. This differential current consists of a resistive fault current and an additional capacitive leakage current which vectorially add up to the differential current. From the specifications of the normative test setup and test procedure for fulfilling the standards it can be taken that a stepwise increase of the fault current alone has to result in a separation of the inverter from the grid, even with a high leakage current being present. For this purpose, it is necessary to determine the fault current portion in the differential current.
In an inverter, the differential current is usually measured with a summation current transformer whose voltage signal is a measure of the differential current between the phases and the zero conductor of the inverter. Due to the tendency to develop transformer-less inverters with increasing power and bigger dimensions of the photovoltaic installations, the capacitances with regard to earth and thus the leakage currents increase. Due to the use of certain materials for the photovoltaic installation, this tendency is additionally increased. Further, the leakage capacitances, like for example in case of a photovoltaic installation as the DC source, are not constant but change inter alia with rainfall onto the photovoltaic panels. For the purpose of sufficiently sensitively recognizing a step in the fault current, it is thus required to continuously separate the leakage current portion from the differential current.
For the purpose of separating leakage currents from the voltage signal of a summation current transformer, it is known to define all slowly varying currents which are measured by the summation current transformer as leakage currents, and to compensate them by an evaluation software. Occurring steps of the fault current may then be calculated vectorially. In this approach, however, steps in the leakage current are erroneously interpreted as fault currents.
A method and a device for insulation and fault current monitoring in an electric AC power grid are known from WO 98/58432 A1. Here, the differential current between the grid lines which is formed by vectorial addition is measured. An alternating current portion in this differential current is determined. As a second grid value, the grid AC voltage between at least two line conductors or between a line conductor and a potential compensation conductor or a zero conductor is measured. Then, the product of the amplitude of the AC current portion in the differential current and the cosine of the phase angle φ between the two measured grid values is determined as a measure of the resistive fault current. This determination may be executed by a multiplication of the AC current portion in the differential current by a multiplication signal and a successive arithmetic formation of an average value, wherein the multiplication signal corresponds to the AC grid voltage whose effective value is kept constant. In case of expanding this known method to a multi-phase AC power grid, the mentioned steps shall be separately executed for each of the three line conductors in that the AC current portion in the differential current of all three line conductors is measured, in that, further, the three grid AC voltages between each line conductor and a zero conductor or a potential equalization conductor are measured, and in that the respective three products of the amplitude of the alternating current portion of the differential current and the cosine values of the three phase angles φ are determined. While the method disclosed is generally applicable to a single-phase inverter, it turns out that the alternating fault current portion in a differential current which is measured at a multi-phase inverter can not be correctly determined in this way. Even with a single-phase inverter, operation conditions occur in which the alternating fault current portion in the measured differential current is not correctly determined by the known method.
There still is a need of a method for determining a fault current portion in a differential current measured at an inverter and an apparatus for executing such a method, by which at least the alternating fault current portion and desirably also the entire fault current portion in the differential current is correctly determined.